If loading into a container a billet with a diameter slightly smaller than an inside diameter of the container, then pushing the billet in the container by a rear extrusion stem against a die to cause so-called “upsetting”, the billet is crushed and the air between the container and billet is compressed. The method of providing a degassing device for discharging this compressed air from a fixed dummy block side of the extrusion stem to the outside of the container which comprises, for example, to be able to move in an axial direction of the extrusion stem, a ring-shaped seal part which is provided at an extrusion stem-side end face of a container which has a container liner in which a billet is loaded, a seal block which is split in two in a direction crossing an axial direction of the extrusion stem, and a pushing device which, when closing the seal block, can make a side end face of the ring-shaped seal part and an outer circumferential surface of the extrusion stem simultaneously closely contact each other through a seal member which is attached to an abutting surface of the seal block and a seal member which is provided at an extrusion stem-side end face of the seal block and which pushes a seal member which is provided at a container-side end face of the seal block against the ring-shaped seal part and of sealing the inside of the container by a seal material while sucking out and removing the air from the gap between the fixed dummy block outer circumferential surface and the container inner circumference wall surface is disclosed in PLT 1.